merge_tools.toml: add VSCodium as a merge tool

[VSCodium](https://vscodium.com/) is a free/libre distribution of
Microsoft's Visual Studio Code editor, it's functionally more or less
the same, but distributed under a FOSS license, unlike VS Code.

This adds VSCodium as a merge tool.

cli/src/config/merge_tools.toml

@@ -44,10 +44,18 @@ merge-tool-edits-conflict-markers = true
 # https://gist.github.com/ilyagr/5d6339fb7dac5e7ab06fe1561ec62d45
 edit-args = ["-f", "-d", "$left", "$right"]
 
+# if you change the settings for vscode, please do the same for vscodium
 [merge-tools.vscode]
 program = "code"
 merge-args = ["--wait", "--merge", "$left", "$right", "$base", "$output"]
 # VS Code reads the output file and behaves better if it's prepopulated with conflict
 # markers. Unfortunately, it does not seem to be able to output conflict markers when
 # the user only resolves some of the conflicts.
 merge-tool-edits-conflict-markers = true
+
+# free/libre distribution of vscode, functionally more or less the same
+[merge-tools.vscodium]
+program = "codium"
+merge-args = ["--wait", "--merge", "$left", "$right", "$base", "$output"]
+merge-tool-edits-conflict-markers = true
+

docs/config.md

@@ -621,12 +621,12 @@ by `jj resolve`. For example:
 
 ```toml
 # Use merge-tools.meld.merge-args
-ui.merge-editor = "meld"  # Or "vscode" or "kdiff3" or "vimdiff"
+ui.merge-editor = "meld"  # Or "vscode" or "vscodium" or "kdiff3" or "vimdiff"
 # Specify merge-args inline
 ui.merge-editor = ["meld", "$left", "$base", "$right", "-o", "$output"]
 ```
 
-The "vscode", "meld", "kdiff3", and "vimdiff" tools can be used out of the box,
+The "vscode", "vscodium", "meld", "kdiff3", and "vimdiff" tools can be used out of the box,
 as long as they are installed.
 
 Using VS Code as a merge tool works well with VS Code's [Remote